FIG. 1 shows a prior art OFDM receiver 100 for wireless signal processing, where wireless signals are received at antenna 114, are amplified and converted to a baseband quadrature signal by RF front end 112, the output of which is sampled by Analog to Digital Controller (ADC) 102, which has a fixed sampling resolution such as 12 bits, which is sufficient for the dynamic range of signals in a packet. The output of the ADC 102 is furnished to phase correction 104, which removes phase rotations associated with frequency differences between the transmitted signal and the received signal, and this output is provided to a symbol timing function 106, which determines the start of packet 116 and symbol location, and provides this to the baseband processor 110 which demodulates the signals and handles any re-transmission requests for lost packets, thereby generating a data stream as was originally transmitted. The RF front end 112 includes a gain control 118 which is set during the preamble interval for single channel systems, or during the first symbol of a new zone in each frame for MIMO (multiple input multiple output) systems by the gain controller 108, as will be described later. The RF front end gain control 118 is critical because the wireless environment includes large variations in receive signal strength. These signal strength variations can be caused by temporally varying ‘multipath’ reflections due to receiver or inter-path reflector movements that add or subtract generating variations in signal strength or can be caused by switching from a 1×1 antenna configuration zone (subframe) to a MIMO N×N antenna configuration zone (subframe) inside a frame which introduces new signal sources and end points and therefore new sets of ‘multipath’ reflections. Additionally, in the IEEE 802.16e standard, a transmitter may have a variety of subchannel configurations, referred to as ‘zones’, which result in the use of a particular subset of available subcarriers to carry information. Two types of zones are FUSC (Fully Used Sub-Carrier) or PUSC (Partially Used Sub-Carrier, where the particular subcarriers in use may vary. When the receiver moves into or out of a zone operating with a different set of subcarriers, the additional power which is transmitted after joining the zone may result in an abruptly higher or lower power level. When the receiver is operating in an environment where the zones are changing and causing different subcarrier combinations to be present, combined with the more slowly varying multipath reflections, there can be a significant variation in the received signal power as these subcarriers undergo different multipath reflections, and subcarrier power levels, as shown in FIGS. 2 and 3. In the prior art of 802.11a and 802.11g, a fixed gain level could be used for an entire wireless packet based on a preamble signal level, whereas in MIMO systems described in 802.16e, a different approach may be required.
FIG. 2 shows a prior art wireless packet such as an OFDM packet of 802.11a or 802.11g, or 802.11ae. Each packet 200 includes a preamble 202 which identifies the packet type and data rate, followed by a succession of symbols 204 . . . 208, and is followed by a similarly formed subsequent packet 220 shown starting with preamble 210.
FIG. 3 shows waveforms for the receiver of FIG. 1 aligned with the time sequence of the preamble and the symbols of the FIG. 2 packet. Waveform 302 shows the incoming signal level before gain adjustment, and waveform 304 shows the gain control voltage (Vagc) used to change the gain of the RF amplifiers of the RF front end 112 of FIG. 1. In the prior art receiver 100 of FIG. 1, the receiver gain 118 is set to a relatively high gain level shown in FIG. 3 waveform 304 as PRE_PKT_GAIN prior to the arrival of a packet, and then the gain is reduced or increased by the gain controller 108 as determined by examining the energy level of the digitized signals at the output of the ADC 102. The gain controller 108 examines the energy level during the preamble as determined by baseband processor 110 to set the RF front end gain 118, as seen in gain waveform 304, which converges in region 308 to a packet gain PKT_GAIN 310 by examining the ADC digitized range of values, and reducing the gain when the maximum values result in saturation of the ADC, and increasing the gain when the maximum values are less than a threshold of 6 db below saturation, or any other threshold below saturation which provides linear operation of the ADC. Once the gain control voltage 304 (signal 118 of FIG. 1) is set during the packet preamble 202, this gain remains in effect for the duration of the packet until the end of the current packet, whereupon it returns to PRE_PKT_GAIN until the arrival of the next preamble.